In many industries, including oil, paper and pulp, textile, electricity generating and food processing, there is an ever-present problem of contaminated water as a by-product of various processes. In particular, water is often used to aid in the production of oil and gas on offshore platforms. This water is usually pumped into a formation in order to be able to pump oil out. As a result, the water becomes contaminated with oil and solids encountered in the formation, and therefore cannot be disposed of simply by dumping it into the surrounding water. Accordingly, numerous methods and systems have been devised to reduce the contaminant content of this water to a level that allows discharge of the water into the sea.
One such system, disclosed in U.S. Pat. No. 4,255,262, comprises a device that mixes and disperses gas in the form of fine bubbles in liquid in a tank in an attempt to remove contaminants from the liquid flowing through the tank. The gas is induced from the upper section of the tank downward into the liquid in the tank via a draft tube. The gas induction occurs as a portion of the liquid contained in the vessel is recirculated back through the individual cells or compartments using a centrifugal pump. The apparatus uses an electrically-driven mechanical skimmer assembly, which serves to remove contaminant-laden froth that accumulates above the liquid level section of the tank. The tank is rectangular-shaped.
The above-mentioned device suffers from various drawbacks. The skimmers are moderate- to high-maintenance items, particularly when used in corrosive environments such as may be encountered in the oil-producing and chemical industries. Also the rectangular tanks, by virtue of their construction, cannot withstand pressures in excess of 2 oz. per square inch (0.8 kPa) internal. This is particularly disadvantageous especially where system pressures upstream of the oil/water separator are prevalent or where noxious or lethal gases such as hydrogen sulfide are present. Furthermore, the rectangular tanks having the skimmers are limited in volumetric capacity because full utilization of the tank is not allowed. In addition, although these tanks are described as “gas tight”, gas pressures are maintained by continuously venting to the atmosphere, which is a potentially dangerous practice if lethal or flammable gases are present.
U.S. Pat. No. 4,564,457, discloses another system for separating suspended matter from fluid. The device comprises a cylindrical tank having an inlet chamber, a plurality of gasification chambers, and a quiescent outlet chamber. A skim trough is disposed near the top of the tank, and extends the length of the gasification chambers into the outlet chamber. Vertical baffles that separate the individual chambers extend downwardly and are spaced from the bottom of the tank, allowing fluid to flow along the bottom of the tank from the inlet chamber to the outlet chamber. Each gasification chamber is equipped with an eductor nozzle assembly positioned centrally in the lower portion thereof. The nozzle assembly provides for recirculation of fluid pumped from the outlet chamber.
In operation, fluid enters the tank through the inlet chamber, passes successively through each of the gasification chambers, and into the outlet chamber, where a portion of the fluid is drawn off to be recirculated through the eductor nozzles. The balance of the processed fluid exits the outlet chamber for further treatment, discharge or storage, depending upon the application. The recirculated fluid is pumped through the nozzle assembly, each nozzle being fed by a common header supplied by a recirculation pump, and each nozzle being positioned co-centrically in an eductor throat assembly. Each eductor throat assembly is connected to a gas header, supplied by a gas volume in the upper portion of the tank common to the aeration chambers and the outlet chamber. The passage of the fluid at high velocity through the nozzles educts gas into the gasification chambers and the gas rises in the fluid in the form of small bubbles. The gas bubbles collect oil and/or suspended solid contaminants as they rise, forming a contaminant-laden froth at the top of the gasification chambers.
While this device presents definite advantages over that disclosed in U.S. Pat. No. 4,255,262, such as the elimination of the mechanical skimmer and problems associated therewith, the ability to operate at higher internal pressure, and better utilization of available tank volume, it suffers from some disadvantages. For example, flow out of the tank must be interrupted in order to remove the contaminant-laden froth from the tank. This can be disadvantageous when a continuous flow of fluid is desirable. Also, the skim trough extends through the gasification chambers into the outlet chamber, which permits froth to spill into the quiescent outlet chamber and contaminate the effluent. Furthermore, there is no means for removing contaminants that may accumulate at the top of the inlet chamber, and no means for venting gas, which may have been entrained in the influent, which accumulates in the inlet chamber. In addition, there is no means for retaining gas in the vessel when the skim outlet valve is opened, creating a potentially dangerous situation if noxious, lethal or flammable gases are present in the tank.
Another problem that is often encountered with the baffles terminating a distance above the tank bottom is that some portion of influent tends to pass under the baffles without being directed to a high turbulence area and contacted by gas bubbles.
Additionally, the velocity of flow in the degasification chamber is relatively high, which leads to insufficient final oil/water separation.
U.S. Pat. No. 4,782,789 relates to an induced static flotation (ISF) cell having an inlet chamber, a plurality of gasification chambers, and an outlet chamber. Contaminated liquid enters the inlet chamber, passes through the gasification chambers, and exits through the outlet chamber. Gas bubbles are introduced into the bottom of each gasification chamber and attract suspended contaminants and/or oil as they rise. A contaminant-laden froth forms at the top of the cell, and is removed via a first skim trough in the outlet chamber. A liquid level displacement controller maintains the level of fluid in the gasification chambers adjacent and below the top of the first skim trough, the second skim trough being vertically adjustable to account for the difference in specific gravity between the liquid in the outlet chamber and the gasification chambers. A timer pulsing device raises the level periodically to provide additional skimming. Gas is recirculated from the top of the cell for introduction into the bottom of the gasification chambers.
Secondary baffles in the gasification chambers prevent a bypass by the liquid and gas of the turbulent area created by the gas flow above the eductor assemblies that deliver the gas into gasification chambers. By using a pair of baffles in the outlet chamber, the fluid retention time is increased to further improve the liquid/contaminants separation.
An invention related to an apparatus for the removal of suspended matter from a liquid, such as used for treatment of oil-containing water is disclosed in U.S. Pat. No. 4,986,903. A cylindrical, horizontal vessel is divided into a single gasification chamber by a partition that extends through the interior chamber of the vessel and allows fluid communication between the two chambers. A liquid to be treated is introduced through distribution header(s) adjacent a bottom of the vessel and/or an alternative inlet nozzle which is combined with a gas eductor in order to achieve a more intimate mixture of the gas and liquid. The gas eductor has its outlet slightly above the outlet of the distribution headers. The released gas bubbles carry oil and suspended matter towards the upper portion of the vessel, from which the froth is collected through a primary skim collection trough which extends through the gasification chamber and from a vertically adjustable (based on specific gravity) secondary skim collection funnel in the degasification chamber. Skim collection is accomplished through control of the liquid level in the vessel.
U.S. Pat. Nos. 5,011,597 and 5,080,780 also relate to an apparatus for removing suspended matter from liquid. The apparatus has a single cell vertical cylindrical hydraulic flotation vessel that is provided with a separation wall to separate a lower gasification chamber from a middle degasification chamber and an upper gas chamber. A number of alternative arrangements are provided for controlling skim collection through controlling volume of liquid within the vessel and changing the volume of liquid through the use of an adjustable timer which intermittently sends signals to outlet valves of the skim collection outlet or of the treated liquid outlet. The apparatus provides for an alternative arrangement of introducing liquid into the vessel, so as to achieve more intimate mixing of gas and liquid introduced into the vessel.
It would be desirable if an apparatus could be devised to overcome some of the problems in the conventional systems for removing suspended matter from a liquid, particularly in systems used on floating offshore hydrocarbon recovery platforms where the action of the waves upon the apparatus tends to cause the suspended matter to contaminate the recovered water.